Induction heating apparatus



Nov. 15, 1955 A. P. BOcK INDUCTION HEATING APPARATUS 2 SheetsSheet 1 Filed June 26, 1952 525 T1 3 m E5 5 .3 3 5 3.3 m 53 2m $39 *2 53530 52cm 530a K Q 6 3 3 3 E EBE 28 a: i k .6 T K N INVENTOR g shley P. Bock. %ZM

aamza MMM ATTORNEY Nov. 15, 1955 A. P. BocK INDUCTION HEATING APPARATUS 2 Sheets-Sheet 2 Filed June 26, 1952 INVENTOR Ashiey P. Bock. B

Z M ATTORNEY United States Patent INDUCTION HEATING APPARATUS Ashley P. Bock, Catonsville, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 26, 1952, Serial No. 295,643 12 Claims. (Cl. 2110.77)

My invention relates to induction heating apparatus and in particular relates to a novel arrangement of oscillation generator and power amplifier circuits which automatically maintains acceptable tuning between its components during major changes in characteristics of the heated load. I have illustrated its broadly novel features by applying them to an apparatus for high frequency induction heating of tin-plated steel strip to improve its surface brightness.

In certain industrial processes of fairly modern developrnent, of which the heating of tin-plated steel may be taken as an example, long strips of sheet metal are runat high speed from reels through helical coils traversed by high-frequency current and thereby heated briefly to a high temperature by currents induced in them. Single furnaces of this type often use power of over 1000 kilowatts, and are often required to maintain constancy of output temperature with strip-speeds varying over an 8 to 1 range, and strip-cross-sections varying over a 4 to 1 range, in the course of the services required o'fth'emf Conventional practice has been to usefrom one to several oscillation generator'tubes of ratings up to 600 kilowatts each supplyieg heat individually-and directly to the load for this service. The U. S. Patent 2,381,323 to M. P. Vore is one example of this. The'necessary adjustments of power-output to maintain constancy of temperature in the face of the changes of load characteristics already mentioned is effected by reactors in the power-supply lines whose magnetic saturation is automatically con-- trolled in response to load temperature. However, while,

technically satisfactory, the saturated reactors and other circuit components are bulky and costly, and my present apparatus has a number of desirable improvements over the prior art. g

' One object of my invention is, accordingly, to provide an improved system for induction heating by high frequency currents.

Another object of my invention is toprovide an arrangement using electrical discharge tubes for heating induction loads which is simpler and less costly than conventional arrangements for this use.

Still another object of my invention is to provide an arrangement for supplying and controlling power to an induction heating load without the use of large and costly magnetic-saturation reactors.

Yet another object is to provide a system supplying and controlling power to induction heating loads which is more rapid in responding to and correcting for changes in load characteristics than were prior art arrangements.

A further object of my invention is to provide an arrangement employing an oscillation generator and power amplifier supplying an inductive load in which variations in the electrical characteristics of the load automatically vary the frequency of the oscillation generator without the interposition of electronic or other ancillary regulating circuits between the two,

Other objects of my invention will be apparent upon reading the following description taken in connection With the drawings, in which:

Figure l is a block diagram of a system in accordance with my invention for induction heating of a tin-plated steel strip; and

Fig. 2 is a detailed schematic circuit diagram of the less conventional portions of the Fig. l arrangement.

Briefly stated, I attain the above-mentioned objects by providmg an oscillator supplying one or more power amplifiers of the electrical discharge tube type and also supplying substantial heating power directly to the load, so that the steel strip being heated threads the resonant tank-circuit of the oscillator and automatically varies the generated frequency to maintain satisfactory system tuning.

Referring in detail to Fig. 1, a steel strip or ribbon 1 is fed from reels (not shown) through three helices 2, 3, 4 carrying high frequency alternating current supplied respectively by a power oscillator and driver 5 and two power amplifiers 6 and 7 which may be of the electrontube type. The oscillator 5 has a power rating of 200 kilowatts, of which only about one-eighth is supplied to the input circuits of power amplifiers 6 and 7 as a driver for them, and the remainder is supplied to the heating-helix 2. The power-amplifier 6 has a rating of 425 kilowatts and the power-amplifier 7 a rating of 600 kilowatts, so that the entire system is designed to supply loads up to 1200 kilowatts.

The oscillator 5 and power-amplifier 6 are supplied with plate voltage by a direct current power source 8. The power-amplifier 7 is supplied with plate voltage from a direct-current source 9. The respective power amplifiers 6 and 7 are supplied with direct-current bias voltage from regulatable direct-current sources 11 and 12 which are regulated in voltage by a regulator 16. The latter responds to a temperature device 14 which responds to the temperature of the heated strip 1 to send signals to regulator 16. The regulator 16 supplies control power to the bias-devices 11 and 12.

The general mode of operation of the Fig. 1 system is as follows. Travel of strip 1 is started by the motors operating its reels, and power supplies 8, 9 energize the driver oscillator 5 and power-amplifiers 6, 7 to heat strip 1. As soon as strip 1 reaches the predetermined tempera.- ture to which it is desired to heat it, the temperature device acts through regulator set 16 to set the respective bias voltages of power-amplifiers 6 and 7 at such values that they maintain the strip 1 at that temperature. It is common practice to heat in succession in this furnace a number of different strips 1 of different cross-section, welding the front end of the second onto the tail end of the first just as the latter is ready to enter the heaterhelix 2, and without stopping travel of the strip through the furnace. If the incoming strip has different dimensions or otherwise tends to heat to a different temperature than the outgoing, the temperature-device 14 will quickly act through regulator 16 to adjust the bias voltage sources 11 and 12 to such values as to restore the temperature of strip 1 to the standard value.

Any change in dimensions of strip 1 will change the tuning of the tank-circuits of power-amplifiers 6 and 7, and to maintain satisfactory operation their tank circuits would have to be readjusted if the frequency of oscillator 5 remained the same as while the outgoing strip 1 was being heated; and quickly variable returning of these tank circuits would require very expensive control-equipment. However, with my arrangement, the same change in tuning as that of the power amplifier tank circuits occurs automatically in the tank-circuit of oscillator 5 be cause the strip 1 constitutes the core of its inductance element, and the frequency of oscillator 5, therefore,

changes in the correct amount so that the tank-circuits of power-amplifiers 6 and 7 remain satisfactorily tuned.

Referring to Fig. 2 for details of certain less conventional parts of the Fig. 1 block diagram, the dot-dash rectangles indicate the portions of the circuit falling within similarly-numbered blocks in Fig. l. The power oscillator and driver comprises a pair of electronic triodes 21, 22 having their cathodes connected to the grounded negative bus 23 which is common to the direct-current plate voltage sources 8 and 9 of Fig. l. The anodes of tubes 21, 22 are connected in multiple through a choke coil 24 to the lead 25 from the positive terminal of direct-current voltage source 8. A tank circuit comprising the heater-helix 2, capacitors 26, 27 and 28, and adjustable inductor 29 is coupled to the anodes of tubes 21, 22 through a blocking capacitor 31 and is grounded at the junction 32 of the capacitors 26 and 27. The control-electrodes of tubes 21 and 22 are connected to their cathodes through a pair of windings 33, 34 which are inductively linked with the inductor 29, thereby feeding back voltage from the tank circuit through variable rheostats 35, 36 to cause the tubes 21 and 22 to form oscillation generators, the power output of which may be set at any desired value in any Well known way as by adjusting rheostats 35, 36 for example. The cathodes of tubes 21 and 22 are energized from a local power-supply (not shown) through a saturable-core reactor 37 having a saturating winding 38, the current in which may be varied to control the cathode heating-current, and so the current output of oscillator tubes 21, 22. A variable resistor 39 shunting a second saturating winding 41 on the core of reactor 37 varies the cathode heating of oscillator triodes 21, 22, reducing their output when the temperature device 14 shows the strip 1 to be too hot as to reduce the output of power-amplifier 6 in accordance with the description thereof below.

The tubes 21, 22 will generate oscillation current in their tank circuit of a magnitude which may be adjusted by varying the rheostats 35, 36 or the saturating winding 38. The heater helix 2 is designed so that when tubes 21, 22 are operating at their rated full-load current about 175 kilowatts of power will be transferred by induction to a strip of the largest cross section intended to pass through helix 2. The voltage to ground across capacitor 27 is fed through a lead 42 to the control electrodes of the tubes 51, 52 of power-amplifier 6 and the tubes 61, 62, 63 and 64 of power-amplifier 7. Similarly, the voltage to ground across capacitor 26 is fed through a lead 43 to the control-electrodes of tubes 55, 56 of poweramplifier 6 and the tubes 65, 66, 6'7, 68 of power-amplifier 7.

The power-amplifier 6 comprises one pair of electron triodes 51, 52 having their anodes connected through a choke coil 53 to the positive bus 25' from direct-current source 8; and a second pair of triodes 55, 56 having their anodes connected to bus 25 through a choke coil 54. The cathodes of the triodes 51, 52, 55, 56 are connected together to the grounded bus 23 from that same source. A tank circuit comprising heater helix 3, adjustable inductors 57, 58 and capacitors 59 and 59A is coupled between ground and the anodes of triodes 51, 52 by a blocking capacitor 66A; and between ground and the anodes of triodes 55, 56 by a blocking capacitor 608.

A direct current bias voltage is applied to the controlelectrodes of triodes 51, 52, 55, 56 by the direct current generator 11 having its negative pole grounded and its positive pole connected to ground bus 23 through variable resistor 39 and winding 41 as already described, and also connected through rheostat 79 to lead 42 and through rheostat 86 to lead 43. The generator 11 has two field windings, one connected to a local direct-current source and the other connected to the regulator 16 of Fig. 1. When, at anytime, the temperature of strip 1 rises above the desired standard value for any reason, the temperature-device 14 and regulator 16 increase the voltage of bias generator 11 to make the control-electrodes of triodes 51, 52, 55, 56 more negative relative to their cathodes, thereby reducing the current in the tank circuit of those tubes and reducing the heating effect of helix 3 on strip 1. Drop of temperature in strip 1 produces the opposite effect on the voltage of generator 11.

The voltage-drop impressed by capacitor 27 in the tank circuit of oscillator triodes 21, 22 on the control electrodes of power-amplifier triodes 51, 52 through lead 4-2, and the similar effect of capacitor 26 on power-amplifier triodes 55, 52, cause the circulation in the tank circuit of these power-amplifier triodes and heater-helix 3 of greatly amplified current of the oscillator frequency. The rheostats 79, and the variable inductors 57, 58 make it possible to adjust this tank circuit at will, for example to a value which induces heating of 425 kilowatts in strip 1 through helix 3.

The power-amplifier 7 comprises four pairs of electron triodes 61, 62, 63, 64-65, 66 and 67--68 having their cathodes connected together to the grounded negative bus 23 from direct-current source 9 of Fig. 1, and their anodes connected to the positive bus from that source through choke-coils 71, 72, 73, 74. A tank circuit in series with heater-helix 4 and including adjustable inductors 75, 76, 77, 78 and capacitors 81, 82, 83, 84 is coupled between ground and the anodes of the respective triode-pairs 61-62 to 67--68 by blocking capacitors 85, 86, 87, 88. The control-electrodes of the triodes 61, 62,. 63, 64 are connected through blocking capacitors 91, 92 to the lead 42 which thus feeds them the high frequency voltage of capacitor 27 in the tank circuit of oscillator triodes 21, 22. Similarly, the control-electrodes of triodes 65, 66, 67, 68 are connected to receive the voltage across capacitor 26 through lead 43 and blocking capacitors 93, 94. The triodes 61 to 68 thus produce a large circulating current in their tank circuit and heater helix 4.

The direct-current source 12 has its positive pole grounded to lead 23 and its negative pole connected through four rheostats 95, 96, 97, 98 to the respective control-electrodes of the four triode-pairs 61-68. The source 12 has two field windings connected like those of bias source 11 and since its operation is the same as that already described for the latter, except that its output to the strip 1 through helix 4 is adjusted to 600 kilowatts, further description of source 12 here is believed to be unnecessary.

The employment of power-amplifiers to feed the heaterhelices instead of supply the latter directly from the oscillation generators has the advantage that tank circuits of lower Q can be used than those required to render oscillators stable. Thus the size and first cost of both capacitors and inductors for the tank circuits are reduced in the order of 50 percent. Likewise less complex and costly regulators sufiice to maintain constant strip temperature than were needed for the conventional prior art systems in that the sluggishness of their large saturable' reactors required forcing auxiliaries in the regulators which are not needed with my above-described system.

A further feature of the instant arrangement is that at full loads the control-electrode circuits of the poweramplifier' actually feed power into the bias sources 11 and 12 and if these are, as is usual, driven by motors connected to the low frequency system which powers the sources 8 and 9, the control-electrode circuits actually return power to the low-frequency supply. This means that the control-electrode drive system has approximately twice the efficiency of the control-electrode drive where all heating-current is supplied direct from power oscillators.

I. claim as my invention:

1. In an induction heater for heating a strip of magnetic' material having a varying cross-sectional area, the combination of an oscillator having afirst tank circuit for tuning the oscillator, with a first coil in said first tank circuit of said oscillator through which said strip of magnetic material to 'be heated passes, a power-amplifier having its input coupled to draw power from said oscillator and, having a second tank circuit with a second coil, with said strip being operable to pass through said second coil of said power-amplifier, with said varying cross-sectional area being operable to effect like changes in the tuning of said first and second tank circuits.

2. In an induction heater for heating a strip of magnetic material having a varying cross-sectional area, the combination of an oscillator having a first tank circuit for tuning the oscillator, with a first coil in said first tankcircuit through which said strip of magnetic material to be heated passes, a power amplifier having its input coupled to draw power from said oscillator and having a second tank circuitincluding a second coil, with said strip being operableto pass through said second coil and draw power from said power-amplifier, and means responsive to the temperature of said load to vary the bias on said power-amplifier, with said varying cross sectional area of said strip being operable to effect like changes in the tuning of said first and second tank circuits.

3. In an induction heater for heating a strip of magnetic material having a varying cross-sectional area, the combination of an oscillator having a first tank circuit for tuning the oscillator, with a first coil in said first tank circuit through which said strip of magnetic material to be heated passes, a power-amplifier having its input coupled to draw power from .said oscillator and having a second tank circuit including a second coil, with said strip being operable to pass through said second coil and be inductively coupled to the output of said power-amplifier, with said varying cross-sectional area of said strip being operable to effect like changes in the tuning of said first tank circuit of the oscillator and said second tank circuit of the power amplifier, and means responsive to a temperature condition of said material connected to vary the power output of said power-amplifier.

4. In a high frequency heater system for heating a load having a varying cross-sectional area, the combination of an oscillator having a first tunable tank circuit, with said first tank circuit being of the type in which its fre quency is determined by an inductive property of said load, a power amplifier having an input circuit and an output circuit, said power-amplifier having its input circuit coupled to said oscillator, said output circuit of said power-amplifier embodying means for transmitting heat to said load and including a second tunable tank circuit of the type in which its frequency is determined by an inductive property of said load, with said varying crosssectional area of said load being operable to effect like tuning of said first and second tank circuits to the same frequency. 7

5. In a high frequency heater system for heating a load having a varying cross-sectional area, the combination of an oscillator of the type in which its frequency is deter mined by an inductive property of the load, a power amplifier having an input circuit and an output circuit, said power-amplifier having its input circuits coupled to said oscillator, said output circuit of said power-amplifier embodying means for transmitting heat to said load and including a second tunable tank circuit of the type in which its frequency is determined by an inductive property of said load, with said varying cross-sectional area of said load being operable to effect like tuning of said first and second tank circuits to the same frequency, and means responsive to a temperature condition of said load to vary the power output of said power-amplifier.

6. In an induction heat apparatus for heating a striplike load having a varying cross-sectional area, the combination of an oscillator having an output circuit, a power amplifier having an input circuit and an output circuit, with'saidpower amplifier having its input circuit coupled to said output circuit of the oscillator, with said output circuit of said amplifier including a first tank circuit for heating the load, said output circuit of said oscillator including a second tank circuit, said second tank circuit havingits frequency determined by an inductive property of the load and thereby effecting the tuned frequency of the output circuit of the oscillator to be the same as the tuned frequency of the output circuit of said poweramplifier.

7. In an induction heater apparatus for heating a striplike load having a varying cross-sectional area, the combination of an oscillator having an output circuit, a power-amplifier having an input circuit and an output circuit, with said power-amplifier having its input circuit coupled to said output circuit of the oscillator, with said output circuit of said amplifier including a first tank circuit for heating the load, said output circuit of said oscillator including a second tank circuit, with said second tankcircuit having its frequency determined by an inductive property of the load and thereby effecting the tuned frequency of the output circuit of the oscillator to be the same as the tuned frequency of the output circuit of said power-amplifier, and means responsive to a temperature condition of said load to vary the output of the power-amplifier.

8. In an induction heater for heating a strip of magnetic material having a varying cross-sectional area, the combination of an oscillator having a first tank circuit for tuning the oscillator, a first coil included in said first tank circuit through which said strip of magnetic material to be heated passes, a power-amplifier having its input coupled to draw power from said oscillator and having a second tank circuit with a second coil, with said strip being operable to pass through said second coil of said power-amplifier, with said varying cross-sectional area being operable to effect like changes in the tuning of said first and second tank circuit, and means responsive to a temperature condition of said load to vary a control-bias voltage on said input circuit of said power-amplifier.

9. In an induction heater for heating a strip-like load having a cross section of varying dimensions, the combination of an oscillator having a frequency-determining first tank circuit, with said first tank circuit embodying a first inductor through which said body to be heated passes, a power amplifier having its input coupled to draw power from said oscillator and having a second tank circuit embodying a second inductor through which said magnetic body passes in the output of said power amplifier with said varying cross-sectional area being operable to effect like changes in the tuning of said first and second tank circuits.

10. In an induction heating apparatus for heat treating a strip-like load having a cross section of varying dimensions, the combination of an oscillator having an output circuit, a power amplifier having an input circuit and an output circuit, said output circuit of the oscillator having a first tunable tank circuit containing a first inductor coil, said output circuit of the power-amplifier having a second tunable tank circuit containing a second inductor coil, with said first and second inductors providing a path for said load to move therethrough, with the tuned frequency of the first and second tank circuits determined by the inductive property of said load, the output of said oscillator being substantially applied to said power-amplifier for driving said power-amplifier.

11. In an induction heating apparatus for heat treating a strip-like load having a cross section of varying dimensions, the combination of an oscillator having an output circuica power amplifier having an input circuit and an output circuit, said output circuit of the oscillator having a first tunable tank circuit containing a first inductor coil, said output circuit of the power-amplifier having a second tunable tank circuit containing a second inductor coil, with said first and second inductors providing a path for said load to move therethrough, with the tuned frequency of the first and second tank circuits determined by the inductive property of the load, the output of said oscillator being substantially applied to said power-amplifier for driving said power-amplifier, and means responsive to a temperature condition of said load, with said means provided to vary a control-bias voltage on said power-amplifier.

12. In an induction heating apparatus for heat treating a strip-like load having a cross section of varying dirnensions, the combination of an oscillator having an output circuit, a power-amplifier having an input circuit and an output circuit, said output circuit of the oscillator having a first tunable tank circuit containing a first inductor coil, said output circuit of the power-amplifier having, a second tunable tank circuit containing a second inductor coil, with, said first and. second inductors providing a path for said load to move therethrough, with the tuned frequency of the first and second tank circuits deterof said oscillator being substantially applied to said power-amplifier for driving said power-amplifier and means responsive to a temperature condition of said had to vary the power output of said power-amplifier.

References Cited in the file of this patent UNITED STATES PATENTS 2,381,057 Hutcheson Aug- 7, 1945 2,381,323 Vore Aug. 7, 1945 2,508,321 Wilmette May 16, 1950 2,570,798 Gullick Oct. 9,, 1951 2,583,227 Neidigh Ian. 22, 1952 FOREIGN PATENTS 605,776 G'erat Britain July 30, 1948 605,777 Great Britain July 30', 1948' 

